The present invention relates to an electronic device including an electrode.
A base of an electrode of an electronic device is generally made of copper since copper is highly conductive and inexpensive. For improving joining of the base to another electronic device, a gold layer, a solder layer, or a tin layer is generally formed on the base. If the gold layer, for example, is formed directly on the copper base, the gold layer changes into an alloy layer composed of copper and gold due to mutual diffusion of gold atoms and copper atoms. The alloy layer causes joining failure in the joining of the electronic device to another device.
For preventing the formation of the alloy layer and the joining failure, a nickel layer is formed between the copper base and the gold layer. The nickel layer is often formed by electroless plating recently since an electrode is small and complicated. Nickel plating solution used for the electroless plating generally contains nickel sulfate and hypophosphorous acid compound as reducer. Nickel is deposited by oxidation-reduction reaction of the nickel sulfate and hypophosphorous acid compound, thereby forming the nickel layer.
However, the oxidation-reduction reaction starts when two components are mixed, and thus, an amount of nickel that does not contributes to the forming of the nickel layer is deposited in the plating solution, thus shortening a life time of the plating solution.
For preventing this problem, a stabilizer for disturbing the oxidation-reduction reaction is added to the plating solution. Catalyst, such as palladium, is applied on the electrode as a pre-treatment before the plating, and then, the electrode is dipped into the plating solution. Thus, the catalyst allows the oxidation-reduction reaction to start, and then, a nickel layer is formed on the electrode.
At this moment, a reduction reaction of hypophosphorous acid, a reducer, occurs as a secondary reaction, and phosphorus produced is taken into the nickel layer. The nickel layer therefore contains the phosphorous.
When the device is soldered on the electrode with tin alloy solder, nickel atoms in the nickel layer diffuse into a solder layer due to heat in the soldering, and therefore, a layer containing relatively-much phosphorus (it is hereinafter called a concentrated-phosphorus layer) is produced at an interface between the nickel layer and the solder layer. The more phosphorus is contained in the nickel layer, the more a metal lattice of nickel metal deforms. As a result, the rate of amorphous material in the nickel layer increases, and property of the nickel metal is tend to decline.
The tendency increases flexural strength of the electrode, but decreases the joining strength between the concentrated-phosphorus layer and the solder layer.
A device includes an electrode including a base, a first layer located over the base made mainly of nickel, and a second layer located on the first layer. The second layer is made mainly of nickel having a phosphorus concentration less than a phosphorus concentration of the first layer.
The electrode has an increased joining strength without having a flexural strength decreased.